1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and method of fabricating thereof, and more particularly, to a polycrystalline LCD device having a large width channel and a method of fabricating the same.
2. Description of the Related Art
There are various kinds of screen display devices displaying image data on a screen. Among those, a thin film type flat-panel display device has been mainly developed because of its advantages of being light weight and portable. Moreover, an LCD device has high resolution and high operating speed to accomplish moving pictures. The LCD device utilizes liquid crystal arranged by an electric field applied thereto to transmit or cut off light by the direction of alignment.
Recently, an active matrix type LCD device has been widely developed to provide excellent picture quality, in which a plurality of pixels are arranged in a matrix form and image data are selectively supplied to each pixel through a switching device such as a thin film transistor (TFT) provided at each pixel. The LCD device includes a TFT array substrate made of a transparent material, for example, a glass material incurring low cost and having high processibility.
Crystalline silicon is utilized as a channel of the transistor because of its high speed operation characteristics. However, if the channel is fabricated with a polycrystalline silicon material having high electron mobility, a switching speed may be high and a size may be designed to be small. But since the polycrystalline silicon is formed through a high temperature process, it cannot be formed on the glass substrate of the LCD device. Therefore, the TFT applied on the glass substrate of the LCD device is made of silicon so that it may be formed through a low temperature process.
Also, the LCD device includes a driving circuit unit that requires numerous switching devices to process digital signals. For this reason, the driving circuit unit is configured with a plurality of integrated circuits (IC) in which small transistors having high switching speed are integrated at high density. Thus, the transistors applied to the driving circuit unit of the LCD device are to be made of the polycrystalline silicon material through the high temperature process.
As mentioned above, the TFT applied to the pixel region of the LCD device is made of the amorphous silicon material, whereas the transistor applied to the driving circuit unit of the LCD device is made of the polycrystalline silicon through the high temperature process. Accordingly, as for the driving circuit unit of the LCD device, a plurality of ICs are individually fabricated on the single-crystalline silicon substrate and then can be mounted on a tape carrier package (TCP) so as to be connected to the substrate of the LCD device in a tape automated bonding (TAB) method, or can be mounted on the substrate of the LCD device so as to be coupled to the substrate in a chip on-glass (COG) method.
However, if the driving circuit unit is coupled to the substrate in the TAP or the COG method, space is required for the driving circuit unit, causing the compact size of the LCD device to increase the simple construction of the device to become more complicated. In addition, various noises or electromagnetic interference (EMI) are generated due to the increase in the number and length of lines transmitting driving signals, resulting in degradation reliability of a product and increase in fabrication unit cost of the LCD device.
Recently, as the research and development for forming the polycrystalline silicon through the low temperature process proceed, the TFT formed on the substrate of the LCD device can be made of the polycrystalline silicon material, and a driving circuit-integrated LCD device has been proposed in which the driving circuit unit can be installed on the substrate of the LCD device.
FIG. 1 is an exemplary view showing a driving circuit-integrated LCD device according to related art. As shown in FIG. 1, an LCD device includes an LCD panel 10 where gate lines 20 are arranged horizontally and at regular intervals, data lines 30 are arranged vertically and at regular intervals, and pixels 40 are formed at square regions sectioned as the gate lines 20 and data lines 30 intersect; a gate driving circuit unit 50 mounted on the LCD panel 10 and applying scan signals to the gate lines 20, and a data driving circuit unit 60 mounted on the LCD panel 10 and applying data signals to the data lines 30.
Each of the pixels 40 includes a pixel electrode and a TFT. The TFT includes a gate electrode connected to the gate line 20, a source electrode connected to the data line 30 and a drain electrode connected to the pixel electrode. Gate pad parts and data pad parts are formed at ends of the gate and data lines 20, 30.
The gate driving circuit unit 50 sequentially applies scan signals to the gate lines 20 through the gate pad parts, and the data driving circuit unit 60 applies data signals to the data lines 30 through the data pad parts to individually drive the pixels 40 of the LCD panel 10, thereby displaying an image on the LCD panel 10. Also, the gate driving circuit unit 50 and the data driving circuit unit 60 mounted on the LCD panel 10 are simultaneously formed during a process of fabricating a TFT array substrate of the LCD panel 10.
As discussed above, since the driving circuit-integrated LCD device has increasingly high resolution and is enlarged, the number of data signals to be processed for driving the LCD device is considerably increased. Accordingly, the driving circuit unit of the LCD device should be driven at a higher speed. However, loads of the data lines and the gate lines are increased so much that it is impossible to apply a desired signal within a short time.
For this reason, a high resolution and large-scale LCD device needs a transistor with a channel having a large width at an output buffer so as to apply a desired signal quickly corresponding to the loads of the data and gate lines. However, the transistor that operates at a high speed and has a large width has a problem that the transistor becomes hot due to the movement of numerous carriers and data processing. In some situations, the temperature of an element may go up to 300° C. so that the element is degraded and cannot be driven normally because of a change in a threshold voltage.
In order to solve this problem, the channel of the transistor with a large width is designed to be sectioned to several parts to promote heat releasing. As shown in FIG. 2, an active layer 100 is formed on a buffer layer of the glass substrate, and includes a plurality of cut-out portions 90 to release heat generated from the channel. A gate line 20 is formed at the central portion of the active layer 100 and supplies a gate scan signal to the channel. Source and drain electrodes 70, 80 are formed at both sides of the active layer 100 to be connected to the active layer 100 through contact holes 70a, 80a. 
The structure of the large-width transistor above is helpful for heat releasing. However, since the structure it utilizes a silicon oxide film, which has very low heat conductivity, as an interlayer insulation layer for separating the gate line 20 and the source and drain electrodes 70, 80, the heat releasing is interrupted when the device is heated. In addition, when devices used for the driving circuit unit become more fast, even the above-described structure of the transistor will have only a limited role in solving the degradation of the devices.